https://www.bilibili.com/video/BV1e24y197Qv/
The classic “Hello World” program looks like this in Java:
public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } } It may only be five lines, but those lines are packed with concepts that are challenging to absorb without already having some programming experience and familiarity with object orientation. Let’s break down the concepts a student confronts when writing their first Java program:
public (on the class). The public accessibility level is relevant only when there is going to be cross-package access; in a simple “Hello World” program, there is only one class, which lives in the unnamed package. They haven’t even written a one-line program yet; the notion of access control — keeping parts of a program from accessing other parts of it — is still way in their future.
class. Our student hasn’t set out to write a class, or model a complex system with objects; they want to write a program. In Java, a program is just a main method in some class, but at this point our student still has no idea what a class is or why they want one.
Methods. Methods are of course a key concept in Java, but the mechanics of methods — parameters, return types, and invocation — are still unfamiliar, and the main method is invoked magically from the java launcher rather than from explicit code.
public (again). Like the class, the main method has to be public, but again this is only relevant when programs are large enough to require packages to organize them.
static. The main method has to be static, and at this point, students have no context for understanding what a static method is or why they want one. Worse, the early exposure to static methods will turn out to be a bad habit that must be later unlearned. Worse still, the fact that the main method is static creates a seam between main and other methods; either they must become static too, or the main method must trampoline to some sort of “instance main” (more ceremony!) And if we get this wrong, we get the dreaded and mystifying "cannot be referenced from a static context" error.
main. The name main has special meaning in a Java program, indicating the starting point of a program, but this specialness hides behind being an ordinary method name. This may contribute to the sense of “so many magic incantations.”
String[]. The parameter to main is an array of strings, which are the arguments that the java launcher collected from the command line. But our first program — likely our first dozen — will not use command-line parameters. Requiring the String[] parameter is, at this point, a mistake waiting to happen, and it will be a long time until this parameter makes sense. Worse, educators may be tempted to explain arrays at this point, which further increases the time-to-first-program.
System.out.println. If you look closely at this incantation, each element in the chain is a different thing — System is a class (what’s a class again?), out is a static field (what’s a field?), and println is an instance method. The only part the student cares about right now is println; the rest of it is an incantation that they do not yet understand in order to get at the behavior they want.
简化后的 hello world
void main() {
println("Hello World");
}
诶哟谢天谢地，恭喜今后的 java 初学者，终于能学一个正常的 hello world 了
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